1. Field of the Invention
The present invention concerns a starting material for injection molding of metal powder, as well as a method of producing sintered parts using such starting material.
2. Description of the Prior Art
Powder metallurgy has been developed as a method of producing those parts having complicated shapes at reduced cost.
As compared with conventional methods using uniaxial pressing, the injection molding method has particularly advantageous features in that it is comparable with the former in view of the mass productivity and can produce those three dimensional structural products of thin-walled small parts that can not be produced by the uni-axial pressing.
In addition, since fine powders can be molded by the use of the injection molding, sintered parts at high density can be obtained. As a result, it is possible to improve mechanical properties, magnetic properties, corrosion resistance, etc.
The injection molding process for a metal powder comprises a kneading step of kneading the metal powder with an organic binder to obtain a starting material for injection molding of the metal powder, a step of applying injection molding to the starting material as in the case of plastic molding thereby obtaining a molded parts, a degreasing step of removing the binder from the molded parts by applying heat treatment, etc. to the molded parts and a step of sintering the debinded molded parts, which are conducted successively.
The process comprising such steps has been known in, for example, Japanese Patent Laid-Open Nos. Sho 57-16103 and Sho 59-229403.
In the above-mentioned technique, however, although the sintering temperature is as high as about 1150.degree. C or above, it is not possible to stably obtain the density ratio of sintered parts (ratio of the apparent density to the theoretical density) of greater than 93%.
Further, none of the disclosed techniques is economically disadvantageous since high sintering temperature has to be applied.
Japanese Patent Laid-Open No. Sho 59-229403 discloses an injection molding method for a mixture comprising a metal powder with an average particle size of greater from 1 to 50 .mu.m and from 35.8 to 60.7% by volume of a binder. However, the density ratio obtained for the powder when sintered at a sintering temperature of 1200.degree. C. for 30 min is only from 82 to 93%.
In view of such situations, it has been demanded for obtaining a starting material for injection molding of a metal powder capable of stably obtaining the density ratio of greater than 93% as well as for the method of producing a sintering product therefrom.